[Impact of resistance of Anopheles gambiae s.s. to permethrin and deltamethrin on the efficacy of impregnated mosquito nets].

Trials to assess the impact of resistance of Anopheles gambiae s.s. to permethrin and deltamethrin on the efficacy of insecticide-treated bednets were carried out from October 1997 to April 1998 in six experimental huts at the Yaokoffikro testing station in Côte d'Ivoire. Six polyester bednets were used. Two bednets were treated with permethrin at a dose of 500 mg/m2 and two with deltamethrin at 25 mg/m2. The remaining two untreated bednets served as controls. The number of Anopheles gambiae s.s. entering the hut was reduced 18% with permethrin-treated bednets and 43% with deltamethrin-treated bednets. Threefold fewer female mosquitoes were found under insecticide-treated bednets than under untreated nets (controls). The number of mosquitoes passing through the treated net was threefold lower. The number of mosquitoes exiting from the treated bednets increased twofold. The blood-feeding rate dropped by 55%. Forty percent of mosquitoes entering the permethrin-treated bednets and 56% entering the deltamethrin-treated bednets died. Immediate mortality was always greater (> 85%) than delayed mortality (< 15%). Bioassays confirmed the results from hut experiments. A lower knockdown effect was recorded with permethrin in the resistant strain. Conversely deltamethrin showed the same knockdown effects in the susceptible (Kisumu) and resistant (Yaokoffikro) strain. Mortality rates were low with both permethrin and deltamethrin. This study shows that, even in areas where Anopheles gambiae s.s. is resistant to permethrin and deltamethrin, bednets treated with these insecticides remain effective and can still be considered as an excellent method of personal protection.